1. Field of the Invention
The present invention relates to a method for surface treating a vacuum brazed heat exchanger, a hydrophilizing treatment agent, and an aluminum heat exchanger.
2. Related Art
Generally, heat exchangers are configured from aluminum or an aluminum alloy, and are made in a complex structure in which fins for exchanging heat are retained between tubes in narrow intervals. For this reason, the surfaces of the fins and the like have been hydrophilized to facilitate the shedding of moisture condensed during air-conditioning. However, the above-mentioned surfaces of fins and the like that have been hydrophilized repeatedly undergo heating and cooling and are exposed to severe conditions in which condensed moisture, dirt in the atmosphere, and microscopic organisms mix and adhere thereto; therefore, there has been a problem in that it is difficult for the hydrophilicity, odor suppressibility, and corrosion resistance to be preserved over an extended time.
The deposits of the brazing filler material can be exemplified as one of the causes of the degradation of heat exchangers. The fins and the like of heat exchangers are commonly assembled by brazing, using a brazing filler material (brazing material) such as aluminum-silicon alloy and aluminum-silicon-magnesium alloy. The method known for normal brazing include a method using a flux for removing the oxide film on the aluminum surface and preventing reoxidation, and a vacuum brazing method (VB method) in which brazing is performed while preventing reoxidation by eliminating the supply of oxygen with a high vacuum. In this way, there is no problem of flux residue since flux is not used in the case of the vacuum brazing method. However, anti-corrosion treatment such as chemical conversion treatment becomes difficult due to the deposits of the brazing filler material adhering to the surface of the fins and the like while brazing, and thus a suitable anti-corrosion coating film cannot be formed. As a result, there have been problems in that the corrosion resistance of the heat exchanger declines, and white rust (oxides of aluminum) forms. Furthermore, there is also a problem in that mold will grow in the moisture absorbed by the white rust, whereby an odor will emanate.
Japanese Unexamined Patent Application Publication No. 2002-30462 discloses a method of performing pickling using ferric salt to effectively remove the deposits of the brazing filler material, then performing a chemical conversion treatment, followed by performing a hydrophilizing treatment. However, the chemical conversion treatment agent used in such a method is primarily a chromate-based chemical conversion treatment agent. In a case of treating a brazed heat exchanger with a chromate-based chemical conversion treatment agent, a favorable chemical conversion coating film can be formed even if some deposits are remaining, and the adherence between the hydrophilic coating film formed thereafter and the chemical conversion coating film can be satisfactorily maintained. However, in recent years, there has been demand not to use chromate-based chemical conversion treatment agents from the viewpoint of the burden on the environment.
Treatment agents that make a heavy metal such a zirconium, titanium, and hafnium as the coating film forming component can be exemplified as non-chromium chemical conversion treatment agents. However, the chemical conversion coating films obtained from the aforementioned such non-chromium chemical conversion treatment agents are inferior in the uniformity of the coating film compared to a coating film obtained using a conventional chromate-based chemical conversion treatment agent, and thus may not be able to obtain a favorable chemical coating film. In particular, in a case of chemical conversion treating a brazed heat exchanger with a non-chromium chemical conversion treatment agent such as a zirconium-based chemical conversion treatment agent, there has been a problem in that an adequate chemical conversion coating film cannot be formed due to the influence of the deposits remaining on the heat exchanger, and thus the adherence with the hydrophilic coating film thus obtained with a conventionally used hydrophilizing treatment agent is lacking. As a result, a problem arises in that degradation of the hydrophilic coating film advances from repeated cooling and heating over an extended time period, and the chemical conversion coating film also degrades accompanying the degradation of the hydrophilic coating film, whereby it becomes difficult to maintain the hydrophilicity, odor suppressibility, and corrosion resistance.
In addition, concerning the odor suppressibility, although Japanese Unexamined Patent Application Publication No. 2002-30462 describes an odor suppressant composed of an organic substance having an amide group and/or phenol group, such an odor suppressant is used for suppressing the foul odor characteristic to the chromate contained in chromate-based chemical conversion treatment agents, and is not offered as any kind of solution to suppressing of the foul odor from the degradation of the hydrophilic coating film or chemical conversion coating film that is characteristic in a case of performing chemical conversion treatment using a non-chromium chemical conversion treatment agent.